Simmons-Smith Reaction

Mechanism of the Simmons-Smith Reaction

Ultrasonication improves the rate of formation of these organozinc compounds,
as with many organometallic reactions occurring at a surface.

The mechanism has not been fully clarified, but pure carbenes can be
excluded, and a metal carbenoid is likely to be involved. The following results
may be interpreted to indicate a possible complexation of the active species by
hydroxy groups leading to reaction on the same face as this substituent. This
would only be possible if an organozinc reagent is present.

Various research groups have developed variants of the Simmons-Smith
cyclopropanating reagent through the replacement of the iodide ligand on the
zinc atom with a strongly electron-withdrawing substituent. Whereas the original
reagent often requires a directing group such as the hydroxyl of an allylic
alcohol, carbenoids such as CF3CO2ZnCH2I and
(PhO)2P(O)OZnCH2I enable a rapid cyclopropanation of
alkenes without the need for such a directing group.